1. Technical Field
The present invention relates to rotors, and in particular, to a magnetic encoded rotor incorporating a means for securing the rotor to a motor drive shaft, the means incorporating one or more cam-type fasteners.
2. Related Art
There are various systems and methods for mounting modular magnetic encoder rotors to a motor drive shaft (or motor shaft). However, there are disadvantages with each of these current systems.
A first system is directed to a rotor having an external hub encircling and extending away from the central aperture. The external hub typically has a diameter less than the outer diameter of the rotor and extends from the face of the hub a relatively short distance. A plurality of set screws is inserted through holes in the hub to engage the motor shaft. Upon tightening, the set screws secure the rotor to the motor shaft. In this embodiment, the external hub is typically positioned external to a stator housing, allowing the rotor to be repositioned after the stator housing is fixed in place. However, a known advantage of a modular encoder is having a thin axial profile, which is defeated by having an external hub. An external hub on a rotor adds thickness to the overall rotor, thereby negating the principal advantage of the modular encoder.
The second method is similar to the first system wherein set screws are used to secure a rotor to the motor shaft. In this embodiment, the set screws are positioned within the working thickness of the rotor such that they engage and tighten against the motor shaft through holes extending from the side edge of the rotor through the rotor's entire thickness. This is possible because of the two tracks normally on the rotor, one of the tracks is typically a “marker” track which utilizes a small portion of the rotor's circumference so that the rest of the circumference on that side is available to locate the set screws. This allows the rotor to maintain its thin axial profile. However, there are several disadvantages with this rotor design. First, because the magnetic encoder rotor is located inside the stator housing, the side edge of the rotor is inaccessible after mounting the stator housing. A rotor cannot be repositioned, repaired, or replaced without removing the stator housing. Second, it is more difficult to reliably achieve maximum clamping force with set screw tightening tools.
The third method is a rotor that is mostly hollow, having an internal hub with integral spokes connecting the internal hub with the outer edge of the rotor. Typically two or three spokes connect the outer diameter to the inner diameter wherein the inner diameter matches the diameter of the motor shaft. One of the spokes has a slot in the center, which extends down through the inner diameter hub. This slot provides the extra space needed to fit the rotor on the motor shaft. A screw passes through the spoke with the slot such that upon tightening, the screw pinches the two halves of the slotted spoke together. This pinching action constricts the internal hub, tightening the rotor to the motor shaft. This system also has disadvantages. For example, it is more expensive to manufacture than using simple set screws. Further, the clamping forces are inferior to those of set screw methods. Lastly, the size of motor shaft diameters that can be accommodated are limited by the fact that the space between the slotted spoke and an adjacent spoke must be wide enough for a wrench or other tightening tool to fit.
A fourth system and method for mounting a rotor to a motor shaft is a rotor having a tapered hub and is divided into two halves. This system then uses a split ring to tighten the hub around a motor shaft. A first, or female, half of the rotor has an inner diameter matching the motor shaft with one side having a tapered groove to the hub defining the inner diameter. A second, or male, half of the rotor is machined with a groove having a taper inverse of the first half. The split ring is contained within the tapered groove of this second half. Then, as the two halves are bolted together with screws, the split ring contained within the halves constricts, thereby tightening the hub of the rotor around the motor shaft. As with other designs, this system for mounting rotor on a motor shaft is more expensive than a set screw system. Also, in the best case, this system has a clamping force comparable to set screw systems. However, this system is subject to assembly errors. If the clamping screws are not tightened uniformly (or if the screw over the split in the split ring) is tightened first, the clamping force is severely limited. There may be no immediate external indication of improper clamping, but such an improperly clamped rotor will slip during normal operation and cause system failures.
Therefore, there is a need for a simplistic system and method for securely mounting a rotor on a motor shaft. There is a further need for such a mounting system to provide easy and quick access to the fastening screws, or other fastening means, thereby minimizing maintenance time. There is still a further need for a mounting system for a rotor in which the rotor retains a thin profile while maintaining maximum clamping force on the motor shaft.